1. Field of the Invention
The present invention relates to heat transfer fluids, sometimes designated "functional fluids." More particularly, the invention relates to heat transfer fluids which have exceptionally high thermal stability.
2. Description of the Relevant Art
In the decade of the 1970's, the energy crisis sensitized industry to the need to conserve our energy resources to a degree never seen previously. The result of this movement toward energy conservation has not slackened despite the more ready availability of energy. Today, most industrial processes are designed to conserve or reuse raw materials and to minimize heat loss. For this purpose, a wide variety of heat transfer fluids are available.
For low temperature heat, the fluorochlorohydrocarbons serve as efficient means of heat transfer. For moderate temperatures, aqueous systems, generally containing suitable corrosion inhibitors are in general use, while at moderately elevated temperatures various glycols such as ethylene glycol, propylene glycol and their low molecular weight oligomers such as diethylene glycol have proven useful and cost effective. For higher temperature applications, however, especially those above 350.degree. F. (176.degree. C.), few satisfactory heat transfer fluids are available, and those that are available suffer from serious drawbacks.
Polyhalogenated hydrocarbons such as polychlorinated and polybrominated biphenyls and biphenyloxides have been utilized as high temperature heat transfer fluids. These fluids are particularly useful where resistance to flammability is important. However, these fluids are expensive, at least potentially carcinogenic, and especially deleterious to the environment. They cannot be utilized in open systems due to the possibility of inhalation. Moreover, these compounds, because of their high halogen content, may cause corrosion problems with certain metals such as the non-stainless steels, copper, lead, and tin at the elevated temperatures at which they are used. Furthermore, in many applications water solubility is essential and these types of fluid are virtually insoluble in water.
Low molecular weight oligomeric polyoxyalkylene glycols are water soluble and can be utilized to some extent in higher temperature applications in closed systems, but tend over long periods of time to resinify and deposit large quantities of tar or sludge. Furthermore, they cannot be utilized in open systems due to their tendency to volatilize and to smoke or fume.
High molecular weight polyoxyalkylene glycols have been proposed as high temperature heat transfer fluids due to their lower volatility and greater thermal stability, for example, in U.S. Pat. No. 3,054,174. Polymers of this class, such as the high molecular weight polyoxyethylene glycols, have achieved some success commercially. However, they are still much too volatile for many high temperature uses such as reflow of low temperature alloys. Furthermore, they tend to fume excessively, and, although water soluble themselves, tend to deposit a varnish type residue which is difficult, if not impossible, to remove by standard techniques such as water washing. An additional drawback is that the more stable, higher molecular weight products are solids at room temperature and thus present handling problems which render them totally unacceptable in some industries.
Polyoxyethylene-polyoxypropylene block copolymers derived from bisphenol A have been proposed for limited use in a high temperature application in U.S. Pat. No. 4,360,144. These bisphenol A initiated copolymers are suggested for use in wave soldering machines where their function is to lower the solder surface tension and minimize surface oxidation of the molten solder. The thermal stability of these prior art polyethers decreased as the polyoxyethylene content increased. However, even in this special application, these polyethylene-polyoxypropylene copolymers have not been able to displace the more economical nonylphenol oxyethylates. Both the nonylphenoloxyethylates and the products of U.S. Pat. No. 4,360,144 have the further drawback of producing sludge during their use, especially when used in bulk. Furthermore, their low water solubility and high viscosity make it difficult to remove them completely from substrates by commercial rinsing techniques.